An OLED display panel is generally comprised of an array of organic light emitting diodes (OLEDs) that have carbon-based films or other organic material films between two charged electrodes, generally a metallic cathode and a transparent anode typically being glass. Generally, the organic material films are comprised of a hole-injection layer, a hole-transport layer, an emissive layer and an electron-transport layer. When voltage is applied to the OLED cell, the injected positive and negative charges recombine in the emissive layer and create electro-luminescent light. Unlike liquid crystal displays (LCDs) that require backlighting, OLED displays are self-emissive devices—they emit light rather than modulate transmitted or reflected light. Accordingly, OLEDs are brighter, thinner, faster and lighter than LCDs, and use less power, offer higher contrast and are cheaper to manufacture.
An OLED display panel is driven by a driver including a row driver and a column driver. A row driver typically selects a row of OLEDs in the display panel, and the column driver provides driving current to one or more of the OLEDs in the selected row to light the selected OLEDs according to the display data.
Conventional OLED display panels have the shortcoming that crosstalk is generated in the OLED display panel. The problem of crosstalk in conventional OLED display panels will be explained in more detail below with reference to FIG. 1.
FIG. 1 illustrates a conventional OLED display panel driven by a conventional driver. The OLED display panel 100 comprises an array of OLEDs 102 coupled between the rows (ROW(n−1), ROW(n), ROW(n+1), ROW (n+2) . . . ) and columns (C(n−1), C(n), C(n+1), C(n+2), . . . ) of the OLED display panel 100. The anodes of the OLEDs 102 are coupled to the columns and the cathodes of the OLEDs 102 are coupled to the rows of the display panel 100. Each OLED 102 has parasitic capacitance 103 associated with it. The parasitic capacitance 103 becomes larger when the associated OLED 102 is not lit, while the parasitic capacitance 103 becomes lower when the associated OLED 102 is lit and current flows through the OLED 102. The OLED display panel 100 is driven by a driver including a row driver 120 and a column driver 140.
The row driver 120 includes row driver control circuitry (not shown) configured to couple the cathodes of the OLEDs associated with a row ( . . . ROW(n−1), ROW(n), ROW(n+1), ROW(n+2) . . . ) of the display panel 100 to either a low voltage (e.g., GND) via resistors ( . . . RL(n−1), RL(n), RL(n+1), RL(n) . . . ) by closing the switches 126 and opening the switches 124 to select the row or to a high voltage (e.g., VCC) by closing the switches 124 and opening the switches 126 to unselect the row. For example, in FIG. 1, ROW(n) is shown selected with the switch 126 associated with ROW(n) being closed to couple ROW(n) to GND through the resistor RL(n) and the switch 124 associated with ROW(n) being open. The selection of ROW(n) by the row driver 120 forward-biases the OLEDs 102 coupled to ROW(n) to light the pixels of the OLED display panel 100 associated with the forward-biased OLEDs 102. Although one OLED 102 is shown for each pixel in FIG. 1, color OLED display panels may have three OLEDs 102 for each pixel, for R (Red), G (Green), and B (Black) and the amount of current through the three R, G, B OLEDs 102 may be separately controlled by separate column driver circuitry like the column driver 140 shown in FIG. 1
The column driver 140 includes current sources 142 that provide current ( . . . I(n−1), I(n), I(n+1), and I(n+2) . . . ) to the columns (C(n−1), C(n), C(n+1), C(n+2) . . . ) of the OLED display panel 100 to drive the OLEDs 102 on the columns. Once a row is selected by the row driver 120, the current sources 142 of the column driver 140 generate current ( . . . I(n−1), I(n), I(n+1), and I(n+2) . . . ) for the corresponding columns (C(n−1), C (n), C(n+1), C(n+2) . . . ) according to the corresponding display data ( . . . Idata(n−1), Idata(n), Idata(n+1), Idata(n+2) . . . ) to drives the OLEDs 102 on the selected row. The amount of current ( . . . I(n−1), I(n), I(n+1), and I(n+2) . . . ) is typically generated to be multiples of a unit driving current (e.g., Iw) and proportional to the display data ( . . . Idata(n−1), Idata(n), Idata(n+1), Idata(n+2) . . . ).
In one embodiment, the display data may be 1-bit data indicating 2 levels of brightness, for example, bright (“1”) or dark (“0”), of the OLEDs 102. Thus, the current ( . . . I(n−1), I(n), I(n+1), I(n+2) . . . ) from the current sources 142 is generated to be, for example, 0 or Iw. In another embodiment, the display data may be 2-bit data indicating 4 levels of brightness, for example, very dark (“0”), dark (“1”), bright (“2), and very bright (“3”), of the OLEDs 102. Thus, the current ( . . . I(n−1), I(n), I(n+1), I(n+2) . . . ) from the current sources 142 is generated to be, for example, 0 or Iw, 2×Iw, or 3×Iw. The OLEDs 102 in the selected row (e.g., ROW(n)) are lit (Iw, 2×Iw, or 3×Iw) or unlit (zero current) based upon the current ( . . . I(n−1), I(n), I(n+1), and I(n+2) . . . ) corresponding to the columns (C(n−1), C(n), C(n+1), C(n+2) . . . ) of the panel 100.
FIG. 2 illustrates the column driving current waveform 202 for one of the columns of the OLED display panel 100 in a conventional OLED driver. As shown in FIG. 2, the column driving current 202 is high during the display scan period 204 with an amount of current proportional to the gray current level as indicated by the display data, and is low during the remaining period of a 1-line display period 206. Note that in a conventional OLED driver, the length of the display scan period 204 is identical for each row of the OLED display panel 100 regardless of the display data for the columns on each row.
Referring back to FIG. 1, there are two types of cross-talks that may be generated in an OLED display panel 100, so-called “bright crosstalk” and “dark” crosstalk.” Bright crosstalk refers to the phenomenon that the lit OLEDs on rows with more black (unlit) pixels (OLEDs) tend to be lit brighter than the lit OLEDs on rows with less black (unlit) pixels (OLEDs). Dark crosstalk refers to the opposite of bright crosstalk, i.e., the phenomenon that the lit OLEDs on rows with more black (unlit) pixels (OLEDs) tend to be lit darker than the lit OLEDs on rows with less black (unlit) pixels (OLEDs).
Bright crosstalk is caused by the difference in the sink current of each row of the OLED display panel 100. As can be seen from FIG. 1, the sink current (Isink(n)) of a selected row (ROW(n)) is determined by the sum of the current ( . . . I(n−1), I(n), I(n+1), I(n+2) . . . ) driving the columns (C(n−1), C(n), C(n+1), C(n+2) . . . ) of the selected row (ROW(n)), which in turn is determined by the display data ( . . . Idata(n−1), Idata(n), Idata(n+1), Idata(n+2) . . . ). Therefore, the sink voltage Vsink(n) across the resistor RL(n) coupled to the selected row ROW(n) is also determined by the display data . . . Idata(n−1), Idata(n), Idata(n+1), Idata(n+2) . . . ), since Vsink(n)=Isink(n)×RL(n). This means that the sink voltages Vsink for the rows of the panel 100 are different from each other, since the column display data varies from row to row.
FIGS. 3A and 3B are diagrams illustrating the bright crosstalk phenomenon. As shown in FIGS. 3A and 3B, each of the columns is driven by a unit current source Iw. In the example of FIG. 3A, the display data is configured to make the region 302 of the panel 100 “black” while making the remaining areas 304, 306, 308, 310, 312, 324 “white.” Assuming 2-bit display data (0 or 1), the current Iw will flow through the OLEDs coupled between rows ROW(n−1), ROW(n+1), ROW(n+2), ROW(n+3) and every column to light the OLEDs on these rows. In contrast, the current Iw will flow through the OLEDs coupled between row ROW(n) and the columns in regions 306, 308 to light the OLEDs but not between row ROW(n) and the columns in region 302. Therefore, the sink current Isink(n) for ROW(n) will be smaller than the sink current for other rows ROW(n−1), ROW(n+1), ROW(n+2), ROW(n+3), causing the sink voltage Vsink(n) for ROW(n) likewise smaller than the sink current for other rows ROW(n−1), ROW(n+1), ROW(n+2), ROW(n+3). As a result, the forward-bias voltage for the OLEDs on row ROW(n) is greater than the forward-bias voltages for the OLEDs on other rows ROW(n−1), ROW(n+1), ROW(n+2), ROW(n+3), causing the white regions 306, 308 to be brighter than the other white regions 304, 310, 312, 314., hence the term “bright crosstalk.”
In the example of FIG. 3B, the display data is configured to make the regions 316, 318, 320, 322, 324 of the panel 100 “black” while making the remaining areas 326, 328, 330, 332, 334 “white.” Because the area of the black regions 316, 318, 320, 322, 324 are different, the sink current Isink(n) will be the largest for row ROW(n+3) and the smallest for row ROW(n−1), gradually decreasing in the rows ROW(n+2), ROW(n+1), and ROW(n) in that order. As a result, the forward-bias voltage for the OLEDs on row ROW(n−1) is greatest and then gradually decreasing in rows ROW(n), ROW(n+1), ROW(n+2), and ROW(n+3) in that order causing the white regions 326, 328, 330, 332, 334 to become darker in that order in accordance with such forward-bias voltage. For example, regions 326, 328, 330, 332, 334 may display brightest white, bright white, white, dark white, darkest white, respectively, hence the term “bright crosstalk”
Referring back to FIG. 1, dark crosstalk is caused by the difference in the amount of parasitic capacitances 103 associated with the OLEDs 102 depending upon the display data for each row. The parasitic capacitance 103 associated with an OLED 102 is larger when the OLED 102 is not lit than when the OLED 102 is lit, because a conducting OLED 102 reduces the associated parasitic capacitance 103. Therefore, a row with more OLEDs unlit will have a larger sum of parasitic capacitance than a row with less OLEDs unlit. Because the row with larger parasitic capacitance has a larger time constant (R-C time constant) and it takes longer to drive the OLEDs 102 associated with such row with a larger time constant, the OLEDs 102 associated with such row with a larger time constant show a reduced brightness even when they are lit.
FIGS. 3C and 3D are diagrams illustrating the dark crosstalk phenomenon. As shown in FIGS. 3C and 3D, each of the columns is driven by a unit current source Iw. In the example of FIG. 3C, the display data is configured to make the region 350 of the panel 100 “black” while making the remaining areas 352, 354, 356, 358, 360, 362 “white.” Assuming a 2-bit display data (0 or 1), the current Iw will flow through the OLEDs coupled between rows ROW(n−1), ROW(n+1), ROW(n+2), ROW(n+3) and every column to light the OLEDs on these rows. In contrast, the current Iw will flow through the OLEDs coupled between row ROW(n) and the columns in regions 354, 356 to light the OLEDs but not between row ROW(n) and the columns in region 350. Therefore, the total parasitic capacitance for row ROW(n) will be larger than the total parasitic capacitance of the rows ROW(n−1), ROW(n+1), ROW(n+2), ROW(n+3). Therefore, it will take longer to drive the OLEDs on row ROW(n) than it would take to drive the OLEDs on rows ROW(n−1), ROW(n+1), ROW(n+2), ROW(n+3), and thus the OLEDs in regions 354, 356 display a darker white than the other white regions 352, 358, 360, 362, hence the term “dark crosstalk.”
In the example of FIG. 3D, the display data is configured to make the regions 374, 376, 378, 380, 382 of the panel 100 “white” while making the remaining areas 364, 366, 368, 370, 372 “black.” Because the area of the black regions 364, 366, 368, 370, 372 are different, the parasitic capacitance associated with row ROW(n+3) will be the smallest and the largest for row ROW(n−1), gradually increasing in the rows ROW(n+2), ROW(n+1), and ROW(n) in that order. As a result, it will take the longest amount of time to drive row ROW(n−1) and the shortest amount of time to drive row ROW(n+3), the amount of time to drive gradually decreasing in rows ROW(n), ROW(n+1), ROW(n+2), and ROW(n+3) in that order, causing the white regions 374, 376, 378, 380, 382 to become darker in accordance with such parasitic capacitance and the associated amount of time taken to drive the row. For example, regions 382, 380, 378, 376, 374 may display brightest white, bright white, white, dark white, darkest white, respectively.
Either one of the bright crosstalk and the dark crosstalk may be corrected by appropriately adjusting the supply voltage VCC powering the column driver circuitry 140. For example, dark crosstalk tends to be more prevalent at lower gray scales, and thus a higher VCC may be used to more quickly charge the parasitic capacitance and thus alleviate the dark crosstalk. However, this will aggravate the bright crosstalk that manifests itself more evidently at high gray scales. In contrast, the bright crosstalk tends to be more prevalent at higher gray scales, and thus a lower VCC may be used to reduce the differences in sink current and sink voltage for each row and thus alleviate the bright crosstalk. However, this will aggravate the dark crosstalk that manifests itself more evidently at lower gray scales.
Therefore, there is a need for an OLED display panel driver that can correct bright crosstalk as well as dark crosstalk.